The lungs of mammals are known to contain endocrine cells and neuroepithelial bodies in the pulmonary epithelium. Neuroendocrine-type granules have been previously identified in both of these structures. Using immunocytochemical techniques we are attempting to identify known polypeptide hormones in normal and neoplastic lung epithelium of various species. Although no adrenocorticotropic hormone (ACTH) was identified in normal lungs, this hormone was identifed in cells obtained from the lungs of Syrian golden hamsters treated with diethylnitrosamine. These cells were also argyrophilic and contained numerous osmiophilic bodies. Thus, these cells appear to be similar to those observed in human lung small cell carcinomas, which are known to contain several pituitary polypeptide hormones. Extracts of rodent lung squamous cell carcinoma induced by intratracheal injectiins of 3-methylcholanthrene or localized exposure of benz(a)pyrene to tracheal transplants were found to contain ACTH and proACTH immunolike reactivity. Since proACTH has been found in the plasma of patients with squamous cell carcinoma, efforts are being continued to purify this protein from bovine pituitary glands.